The present invention relates generally to the field of non-invasive imaging and more specifically to the field of computed tomography. In particular, the present invention relates to improving image quality by estimating and reducing scatter in an X-ray imaging system.
CT scanners operate by emitting fan-shaped or cone-shaped X-ray beams from an X-ray source towards a detector. The X-ray source emits X-rays at numerous angular positions relative to an object being imaged, such as a patient, which attenuates the X-ray beams as they traverse the object. The attenuated X-ray beams are detected by a set of detector elements, which produce signals representing the attenuation of the incident X-ray beams. The signals are processed to produce data corresponding to the line integrals of the attenuation coefficients of the object along X-ray paths connecting the source and detector elements. These signals are typically called “projection data” or just “projections”. By using reconstruction techniques, such as filtered backprojection, useful images may be formulated from the projections. The images may in turn be associated to form a volume rendering of a region of interest. In a medical context, pathologies or other structures of interest may then be located or identified from the reconstructed images or rendered volume.
It is generally desirable to develop CT scanners with high spatial and temporal resolution, good image quality, and good coverage along the z-axis, i.e., the longitudinal or rotational axis of the CT scanner. To meet some or all of these objectives, it may be desirable to increase the coverage provided by the detector, thereby allowing greater scan coverage in one or more dimensions. For example, z-axis coverage of the detector may be lengthened by increasing the number of rows of detector elements in the detector.
However, various physical factors associated with the X-ray imaging process may lead to artifacts in the resulting images or to blurring or generally poor image quality. For example, X-rays photons emitted through the imaging volume may pass through the patient or other object being imaged or be absorbed by the patient or object and thus never reach the detector. The amounts of X-ray photons passing through the patient and the amount attenuated are useful to produce the desired radiographic images as this information is indicative of the composition and structure of the patient or object undergoing imaging. At operating voltages of typical X-ray systems, less than 1 megavolt, three dominate absorption processes contribute to the mass attenuation coefficient of the object: photoelectric absorption, Rayleigh scattering, and Compton scattering. Photoelectric absorption is a mechanism where the energy of the photon is absorbed by the material's electrons and liberated. Rayleigh scattering is an interaction between the photon and material's electrons, where the photon direction is slightly altered, without any loss of energy. Compton scatter is an interaction where the material absorbs part of the energy of the photon; however, the photon continues to traverse the object or patient along an altered direction. Unlike X-ray photons that are photo-electrically absorbed or undergo Rayleigh scattering, an X-ray photon that is attenuated by the Compton scattering mechanism may eventually reach the detector apparatus but typically along a different trajectory. As a result, a scattered X-ray photon may impact the detector at a location or from a direction that conveys no useful composition or structural information about the patient or object undergoing imaging. As a result, the scattered X-ray photons may lead to blur within the resulting radiographic image or otherwise reduce the image quality, such as CT number nonuniformity or a reduction in the contrast-to-noise ratio in a reconstructed image. The likelihood of such scattering may be increased in imaging systems employing multiple X-ray sources or emission points or increased coverage on the patient or object being imaged.
In order to reduce scatter, collimators or anti-scatter grids may be used, which are focally aligned to the X-ray beams from the sources to the detector elements, with a corresponding increase in mechanical complexity and cost of the overall CT system. Further, use of collimators with higher resolution detectors has proven challenging due to the small size of the detector elements or pixels. An alternative method of estimating scatter by attempting to extrapolate scatter signals from detector elements at opposing lateral sides of the detector array has proven difficult and does not provide a reliable estimate for scatter across the full axial volume. A technique for reducing scatter in X-ray imaging while reducing the mechanical complexity and cost of the imaging system is therefore desirable.